Load Calculation Backup Generator

I'm thinking about buying an 8000W, 240V, portable, backup generator to connect to my house when the power goes out. This type of generator,
obviously, is designed to be used with extension cords that would be routed through an open window or door, for example. However, I would like to connect it directly to the service entrance on the back of my house. I am aware that this type of connection requires a "transfer switch".
My question is, do I have to worry about how much of a load I have on each phase of the generator's output? Or, do I only have to worry about total wattage?
To clarify, let's take an extreme (and unrealistic) example. Let's say I have a house that draws 6000W on one phase and 1000W on the other phase and I connect to the service entrance via the generator's 240V, 30A receptacle. So, in this situation I would be drawing about 50 Amps on phase 1 and 8 Amps on phase 2, for a total of 7000W.
Just as an example, there is an 8000 W generator at:
http://tinyurl.com/plg7l
There is a schematic (PDF, page 2) for this generator at:
http://tinyurl.com/ra6kr
This schematic shows a "120/240V 30Amp" socket connector protected by a "30Amp 2P" circuit breaker (CB1). So, I guess I've answered my own question. It looks to me like a current greater than 30A on either pole will trip the breaker.
So, it's back to the drawing board and here's where I'm definitely getting in over my head. Is it possible to parallel the windings somehow so that the two legs are in phase?
If you could do that then the total output could be connected to both bus bars in the circuit breaker box. Now in the (hypothetical) situation where one bus bar draws 6000W and the other draws 1000W, there wouldn't be a problem.
The disadvantage would be that you couldn't operate any 240V appliances. However, with these relatively small generators, you probably wouldn't want to do that anyway. A clothes dryer probably draws 5000-6000 watts all by itself. You would lose the option to operate one or two burners on your electric range, but you could use your microwave oven or your toaster oven instead.
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You are really good at answering your own questions! You are beginning to understand the tradeoffs involved in a standby power system. If you are willing to do a little intelligent load management, and perhaps give up a few of your largest loads entirely, you will do fine.
Now how are you going to fuel this monster?
Vaughn

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On Wed, 10 May 2006 00:00:19 GMT, "Vaughn Simon"
Exactly how our friends in Punta Gorda Florida (Charlie) referred to it. "who is going to feed the monster"? It is not an insignificant amount of fuel either. They were filling up cans every day.
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but none of your 240Vac Loads would operate. Most of the 240Vac Loads wouldn't really be nessesary in Emergency Powered situations anyway.
Bruce in alaska
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I don't talk about it much because few people would understand, so therefore many would assume I was doing something dangerous, or immoral, or unpatriotic... but that is exactly the way my CCK has been wired since the beginning. It is one of the ways that we survive just fine on 4KW (or even less when the Onan fails us).
Vaughn
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Bruce in Alaska wrote:

Bruce In most homes in the US and Canada what you suggest can lead to circuit failure and fire. Most homes contain at least one multiwire branch circuit. In order for the neutral of such a circuit to not overheat and fault out there must be a voltage between each ungrounded conductor and the grounded neutral conductor and a voltage between the ungrounded conductors. When a generator is fed into a homes wiring using a bootleg Edison rather than a true Edison circuit the voltage between the two separate ungrounded current carrying conductors is zero so the current on the shared grounded current carrying neutral conductors is the sum of the current on the two ungrounded current carrying conductors that it serves. In the real Edison Circuit arrangement the the current in the grounded current carrying neutral conductors is only the difference in the current flow of the ungrounded conductors of the multi wire branch circuit. -- Tom Horne
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snipped-for-privacy@yahoo.co.uk wrote:

You've found the wonders of load management. You have a generator that has much lower capacity than your normal service so you have to selectively apply loads to stay within it's capacity.
When you have a power failure you turn off all the circuit breakers in your well labeled (it is well labeled isn't it?) service panel first, then connect and start your generator and then throw the transfer switch to provide generator power to the panel. After that you have to actively manage which circuits you have on.
For instance you probably want to switch on your lighting circuits, but make sure you don't have an excessive number of lights on. This is your small baseline load that you'll keep on. This load probably represents about 5A/leg on your well balanced service panel (it is well balanced isn't it?).
After your baseline load you have a variety of loads that the generator can power individually, but not together. Fortunately these loads are rarely time critical so one can "wait" while another is using the generator capacity.
Examples are:
Heat or A/C - These loads can easily stand to wait even an hour while the generator powers other loads.
Refrigerator - A refrigerator can easily go a couple hours (with minimal opening) without an issue.
Well pump - The pressure tank usually holds at least 20 gal of useable water so you can wash hands and flush toilets a few times before you have to provide power to the well pump.
Stove / oven / microwave - Again you can hold off cooking dinner until the heat or A/C has completed it's cycle and those loads can wait the hour until dinner is cooked.
Water heater - A 50gal+ well insulated water heater will maintain a reasonable water temp without power for a good while.
Of course managing these loads takes a bit of work. In really big installations automated load management is fairly common and there is no reason that such systems can't be scaled and applied to a residential sized installation.
Pete C.
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You can also download Homer from NREL. I use it for my cabin.
http://www.nrel.gov/homer /
Max http://www.northernmichigansolar.com
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I have used an 8000W backup generator during power outages for the house for over 20 years. It was wired into the main panel with a dedicated plug. I strongly recommend an automatic transfer switch, even though I didn't use one.
It's really no problem, once you understand your energy "budget." All the lighting was OK, but we turned off each light as we left the room. The generator ran the furnace, the freezers, fridge and TV without concern. Using the microwave, dishwasher or coffeemaker, required a bit of planning, but worked quite well if you didn't use more than one of them at a time.
We refrained from using the dryer or anything with big electric elements whenever possible. I suspect we could have used them, by turning other things off, but the outages were generally short enough that we didn't need them. The 220V submersible pump in the well was a bit of a concern, but we could generally assume it wasn't running when no one was using water. You could also tell a lot by the sound of load on the engine. The general rule was "one thing at a time, and nothing unnecessary" and everything worked fine.
I use a 10,000 watt generator now, but the same general rules apply.
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BIG WARNING:***************
You MUST rember to shut off the main breaker before the generator is plugged into the house! If the current from the generator flows backward into the powercompany's transformer it will be a sight to see! You may fry a lineman, or blow up the transformer.
Transformers take large voltages and reduce the voltages through differences in windings. for example power goes from big(1k+ volts) to small (115v leg's). They also go in reverse, turning small to big if used improperly!
Be careful!
--Alex
*** end big warning ***
JoeSP wrote:

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On 9 May 2006 22:28:44 -0700, snipped-for-privacy@gmail.com wrote:

While there is a theoretical issue with safety for a lineman, consider that if the power outage is widespread and the OP fails to open the main breaker, he will simply be trying to power the whole grid with his 8 KW. Talk about "load sharing".
He won't be burning up any power company transformers but his APU won't be happy. Of course an automatic transfer switch is in order.
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wrote:

It wasn't very theoretical for the families of these linesmen:
http://www.gulfcoastnews.com/GCNnewsHancockKatrina2.htm
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/15848663/gotomsg/15853602.cfm
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JoeSP wrote:

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/15848663/gotomsg/15853602.cfm
I suspect that the line was down between the house and the local HT transformer, when the lineman picked it up to reconnect it, assuming it would be dead....
I've come across one bright spark who wired the generator to a three pin plug ( yes, with bare pins..) and plugged that in with the house isolator closed. It tripped out the genny. He re-started the genny and then went to reclose the genny trip - but, meanwhile, mains supply had been restored. ..
It just tripped out the genny again.
Genny designers do seem to know what they are doing....
--
Sue






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wrote:

That's utterly ridiculous. It's a well-known fact that generators can backfeed through a transformer and re-energize the line. Lines charged with kilovolts of electricity require very little current to kill someone who touches it.

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On Wed, 10 May 2006 13:53:48 GMT, the renowned "JoeSP"

That's why code in most places requires an approved transfer switch to be installed. Here's one available at Home Despot:
http://www.homedepot.com/prel80/HDUS/EN_US/diy_main/pg_diy.jsp?prod_id 3249&cm_mmc=1hd.com2froogle-_-product_feed-_-D27X-_-163249&srccode=cii_14110944&cpncode-19535279-2
Best regards, Spehro Pefhany
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Spehro Pefhany wrote:

http://www.homedepot.com/prel80/HDUS/EN_US/diy_main/pg_diy.jsp?prod_id 3249&cm_mmc=1hd.com2froogle-_-product_feed-_-D27X-_-163249&srccode=cii_14110944&cpncode-19535279-2
http://www.connecticut-electric.com/default.asp
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JoeSP wrote:

Far from being ridulous, it is one possible scenario. And, generally speaking, the same current is needed to kill someone whether delivered at 120v or 11kV - assuming a series element that limits the current to that value.
Don't US linemen either work using live circuit protocols, or tie the circuits to ground before working on them? The risk of some idiot energising what should be a dead circuit seems so obvious that I find it very hard to believe that they don't..
--
Sue




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Palindr?me wrote:

After the hurricanes here in Central Florida they were driving an area to map out all the downed lines and lines with tree branches stuck in them in a section. They generally worked by splicing the wires that were low enough to touch on the supply side poles first, then when it was safe they energized that section. Finally, they spliced the rest of the downed lines hot. The crew doing restoration in my area was from Texas, and didn't have the right kind of fuses for the 7200 volt line feeding about 100 homes.
They were in fully insulated bucket trucks with HV rated gloves during all of this. Otherwise they would have had to shut down the whole area at the substation and do all the repairs which took weeks. Some power poles with minor damage weren't replaced for over a year. They had red flags and a job number nailed to them to warn workers that they were not safe to work on with anything smaller than a line bucket.
I believe we had more fatalities from traffic accidents than electrocutions. One crew's bucket truck was hit on the highway after they had finished their work, and they were headed back to their home state. :(
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wrote:

Anything to win an argument? Even something so stupid as not having to worry about your generator backfeeding through the lines?
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In the US, Linemen DO follow HOT LINE Protocol when working with ALL Repair type operations. That being Said, when they first come to a WorkSite, dealing with downed lines is a BIG DEAL, and having some DUFUS, backfeeding the grid with his toy genset, is a BIG Concern, and it happens much more often that anyone suspects.
Me been there, had to deal with that......
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